1. Field of the Invention
The present invention relates generally to a dual-mode user equipment (UE) supporting different radio access technologies (hereinafter referred to as “RAT,” and in particular, to a method for performing inter-RAT cell measuring and reporting.
2. Description of the Related Art
GSM (Global System for Mobile communication), a digital mobile telephone system classified as a 2nd generation (2G) cellular system, is widely used in Europe and other areas. GPRS (General Packet Radio Service) developed based on GSM, an improved packet-based mobile communication service, is an improved service for a 3rd generation (3G) mobile communication system, especially a UMTS (Universal Mobile Telecommunications System) system, for providing high-speed, high-quality voice and multimedia services.
The UMTS system, though it is based on the GSM standard, employs W-CDMA (Wideband Code Division Multiple Access) and provides a service in which mobile phone users or computer users can transmit packet-based text and multimedia data at 2 Mbps or higher no matter where the users are located all over the world. The UMTS system employs a virtual connection concept, called “packet switched connection” using a packet protocol such as an Internet protocol (IP), and can always access any other terminals within the network.
Mobile communication technology will evolve from a 2G cellular system, such as GSM/GPRM, into a 3G mobile communication system such as UMTS. However, the evolution from the 2G technology into the 3G technology will be made gradually over quite a long period. Accordingly, a network operator needs to provide a user with a dual-mode user equipment (or dual-mode mobile terminal) that supports both technologies, i.e., GSM/GPRS and UMTS. In particular, when a mobile subscriber moves from one place to another place across several countries where different RATs are serviced in each radio communication network, the mobile subscriber will need the dual-mode user equipment (hereinafter referred to as “UE”). Therefore, in order to succeed in commercializing the 3G technology, a UE capable of operating in more than one radio access technologies is required. Before the UMTS technology is completely commercialized, users will use dual-mode UEs that support both GSM/GPRS and UMTS. By doing so, the users can be provided with a multi-RAT service in which the dual-mode UEs are switched to the currently available technology such as GSM 900 MHz or GSM 1800 MHz even in an area where the UMTS technology is not available.
In order to support seamless mobility between cells of the same RAT or cells of different RATs through handover and cell reselection, the dual-mode UE must perform measurement on a downlink channel. Meanwhile, in order to order a UE to perform cell reselection, packet cell change, and handover, a network requires a measurement report on 2G and 3G cells from the UE. Likewise, the UE also requires measurement values of two RATs for automatic cell reselection between either same RATs or different RATs.
While a UE in an idle mode is moving from GSM/GPRS coverage to UMTS coverage or vice versa, the UE continuously searches for a better cell to enter. In the idle mode, a measurement report on a serving cell and neighboring cells is triggered for reselection, i.e., automatic cell reselection. In the idle mode, cell reselection can be performed on another RAT based on the measurement report by a UE according to which RAT the UE is currently entering. In order to perform cell reselection, a measurement report on a serving cell and neighboring cells is required, and criteria for inter-RAT cell reselection are estimated from the measurement report.
FIG. 1 illustrates a radio wave environment in which a UMTS UE moves from a UMTS region to a GSM region. Referring to FIG. 1, when a UMTS UE located in a UMTS region moves to a GSM region, the UMTS UE will continuously measure a serving cell. The UMTS UE compares measurement results of a serving cell (or UMTS cell) with a RAT threshold. If a UMTS UE moves from a particular place to the GSM region, signals received at the place from UMTS cells will be very weak. As a result of the movement, if GSM cells have higher signal strength than UMTS cells, and cell selection and reselection criteria are matched with each other, the UMTS UE is switched to a GSM cell.
In order to support both GSM/GPRS RAT and UMTS RAT, a dual-mode UE should have a GSM/GPRS protocol stack and a UMTS protocol stack, and in order for the dual-mode UE to support inter-RAT measurement, interaction between the GSM/GPRS protocol stack and the UMTS protocol stack is required. In other words, in order to acquire measurement results on ordered cells, specification regarding how UMTS and GSM will interact is required. A moving UE has information on a serving cell and information on reserve neighboring cells (or measurement target cells), and these cells are called “ordered cells.”
However, a 3GPP (3rd Generation Partnership Project) specification has not provided how inter-RAT measurement should be performed by a multi-RAT UE such as a dual-mode UE.